Generally, bicycle caliper brakes include a pair of brake arms which carry at their utmost ends brake shoes and which are fitted at bosses onto a support bolt; an adjusting nut and a lock nut screw with the utmost end of the support bolt, so that the brake arms are adapted to be rotatable relative to each other.
When the bosses of the brake arms abut directly against each other, frictional resistance between the bosses becomes large, thus hindering smooth relative rotation of each brake arm and reliable braking action.
Accordingly, a caliper brake has hitherto been proposed which interposes a hard washer between the opposite surfaces of the brake arms so as to effect smooth relative rotation of each brake arm as described in Japanese Utility Model Publication Gazetta No. Sho 56-36,700. This caliper brake, however, creates difficulties in adjusting the adjusting nut because the brake arms are supported and tightened on the support bolt through the adjusting nut and a lock nut; the adjusting nut may be tightened excessively and cause larger frictional resistance between the washer and the bosses. As a result, smooth relative rotation of each brake arm may be difficult.
When the caliper brake exerts the braking action, the brake shoes make contact with the wheel rim and the rotation of the wheel acts on each brake arm through the wheel rim and the brake shoes so that the brake arms are biased axially of the support bolt to apply a scooping force onto the support portion of each brake arm at which the brake arms are supported on the support bolt. Hence, the washer used as above-mentioned is subjected to an excessive unbalanced load, thereby creating a problem in that the abutting surfaces between the washer and each brake arm cause not only one-sided wearing but also a seizure, thus resulting in rapid deterioration of the rotational efficiency of each brake arm.
Even when the adjusting nut is well adjusted to smooth the relative rotation of each brake arm, rain water or dust can enter between the opposite surfaces of the bosses at the brake arms, thereby also creating a problem in that the rotational resistance of each brake arm increases, also resulting in rapid deterioration of the rotational efficiency thereof.